Desalting crude oils is a two step process—creating a fine dispersion or emulsion of fresh water and the oil, and then effecting a phase separation of the water from the oil. Smaller water droplets provide more area for mass transfer of ionic contaminants (mostly chloride salts) from the oil into the water than large droplets. In a typical refinery desalter, the dispersion is created by adding water to the raw crude and then passing the mixture through shell and tube heat exchangers and a mix valve. The mix valve typically imposes less than 10 psi (some plants run at up to about 15-18 psi) of pressure drop on the mixture to created a dispersion. The dispersion is then broken and phase separated in a liquid-full large vessel at about 300° F., with residence times of 30 to 40 minutes, and water coalescence is enhanced by chemical demulsifier addition and the imposition of electrostatic fields.
In U.S. Pat. No. 4,415,434, Hargreaves and Hensley describe a multistage process for dedusting and desalting tarsands, shale oils and coals that uses a standard centrifuge to remove dust and solids from an oil-water emulsion. Thacker and Miller describe a similar process in U.S. Pat. No. 4,473,461 for dedusting heavy oil derived from solid hydrocarbon-containing material such as oil shale, coal or tar sand, into purified streams of oil. Goyal, et al., U.S. Pat. No. 5,219,471, use an electrostatic process of blending crude oil with water and desalting chemicals to remove metals and insoluble materials from the crude oil. In Ohsol, et al., U.S. Pat. No. 4,938,876, U.S. Pat. No. 5,882,506 and U.S. Pat. No. 5,948,242, the rag-oil layer, wash water, and fines are mixed to create a single stream from which oil is recovered with a lighter hydrocarbon diluent by demulsification and phase separation. Engel, et al., U.S. Pat. No. 7,612,117, use of a class of acetylenic surfactants to break water and oil emulsions. These processes dissolve, “break” or disrupt emulsions prior to use of centrifugal forces to create separate oil and water phases.
Oil/water separators like air flotation units, dissolved air flotation, the Ohsol separation process, and other separation technologies are well established in the crude oil refining industry. Unfortunately, the industry is averse to changes in technology and new technologies must be well established and proven before implementation. Even with a need for better separation (Nnanna, 2008), developing new technologies is difficult, expensive and very hard to implement and test, especially on a scale comparable to the hundred thousand or more barrels-per-day required for even small refineries.
Standard desalters have worked well historically with the lighter, less viscous crude oils or for separation of solids from crude and shale oil sources, but their performance is challenged by less traditional low-gravity, high viscosity crude oil. Traditional crude oil desalters have poor ionic salt removal as refineries process lower gravity/higher viscosity oils. When working with unconventional oils, pilot scale desalting runs using electrostatic desalters, including electrostatic coalescence technology or electrocoagulation techniques and the Ohsol process for heavy oils were not effective. Because these unconventional crudes generate higher water-in-crude and higher oil-in-water levels than traditional crude oils, improved methods are required to dewater and desalt unconventional and difficult to work with crude oils. What is required is an inexpensive method that can be integrated into current refinery processes for separating crude oil from the desalter rag layer.